Ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with high-energy ions. In semiconductor fabrication, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses and at different energy levels. A specification of the ion species, doses and energies is often referred to as an ion implantation recipe.
FIG. 1 depicts a conventional ion implanter system 100. As is typical for most ion implanter systems, the ion implanter system 100 is housed in a high-vacuum environment. The ion implanter system 100 may comprise an ion source 101 and a complex series of components through which an ion beam 10 passes. The series of components may include, for example, an extraction electrode 102, a suppression electrode 103, a ground electrode 104, a filter magnet 106, an acceleration or deceleration column 108, an analyzer magnet 110, a rotating mass slit 112, a scanner 114, and a corrector magnet 116. Much like a series of optical lenses that manipulate a light beam, the ion implanter components can filter and focus the ion beam 10 before steering it towards a target wafer 120 (located in a wafer plane 12). A number of measurement devices, such as a dose control Faraday cup 118, a traveling Faraday cup 124, and a setup Faraday cup 122, may be used to monitor and control the ion beam conditions.
For a uniform distribution of dopant ions, an ion beam is typically scanned across a surface of a target wafer. FIG. 2 shows a typical setup for continuous wafer implantation with an ion beam. In such a setup, a wafer 204 may flow slowly along an axis 212 through a wafer chamber. At the same time, an ion spot beam 202 having an oval cross-section may be scanned horizontally across a surface of the wafer 204. The scanned ion spot beam 202 may form a beam path 20 that has end points 208 and 210. A dose control Faraday cup 118 may be used to measure current of the ion spot beam 202.
In production, it is desirable to achieve a uniform ion beam profile along a beam path. The process of tuning an ion implanter system to achieve such a uniform ion beam profile is called “uniformity tuning.”
FIG. 3 illustrates a conventional method for uniformity tuning. In a setup similar to the one shown in FIG. 2, an ion spot beam 302 having an oval cross-section is scanned at a constant scan velocity along a path 30 between end points 308 and 310. The ion spot beam 302 is relatively small compared to the size of a wafer 304, which is typical for medium- and high-energy ion beams. In existing uniformity tuning methods for these types of ion beams, it is usually assumed that: (1) the ion spot beam will maintain substantially the same size and deliver substantially the same current during scanning; and (2) the ion spot beam is scanned fully off the wafer edge. It follows from these assumptions that, during scanning, the ion spot beam will only exhibit small changes and any resulting non-uniformity in current density distribution of the scanned ion beam along its beam path is also small. Thus, in existing uniformity tuning methods, a profile of the ion beam is typically only measured once (unscanned) at the center of the wafer. In FIG. 3, a waveform 32 illustrates an individual beam dose density profile for the ion spot beam 302. As the ion spot beam 302 is swept along the path 30, the current density of the ion beam 302 at each point is an accumulated effect of one or more individual beam dose density profiles of the ion spot beam 302. A waveform 34 illustrates a current density distribution of the ion spot beam 302 between the end points 308 and 310. Since the individual beam dose density profile of the ion spot beam 302 is practically the same everywhere along the path 30, there is a linear relationship between the current density and the scan velocity of the ion spot beam 302. That is, for each location in the path 30, a small increase in the scan velocity will cause a proportional decrease in the current density of the ion spot beam 302 at that location, and a small decrease in the scan velocity will cause a proportional increase in the current density of the ion spot beam 302 at that location. Existing uniformity tuning methods are based on this assumption of linearity—if the ion beam current density profile 34 is not uniform enough, a typical approach is to adjust, for each unit distance along the path 30, the scan velocity of the ion spot beam 302 proportionally to the accumulated current density of the ion spot beam 302 within that unit distance.
It should be noted that the current density of the ion spot beam 302 rolls off quickly near both end points 308 and 310. However, since the ion spot beam 302 is scanned fully off the wafer edges, the roll-offs do not affect ion beam coverage of the wafer 304. It should also be noted that, once the ion spot beam 302 has been swept fully off the wafer 304, the current of the ion spot beam 302 no longer contributes to wafer implantation. The ratio between the accumulated ion beam current on the wafer 304 and the total ion beam current accumulated during a full scan is referred to as “beam utilization.” Beam utilization indicates what portion of the total ion beam current is actually utilized for wafer implantation.
In addition, in existing ion implanter systems, uniformity tuning typically follows beam-line tuning. For example, in the ion implanter system 100 shown in FIG. 1, the ion source 101 and one or more of the beam-line elements (e.g., extraction electrode 102, suppression electrode 103, filter magnet 106, acceleration or deceleration column 108, analyzer magnet 110, scanner 114, or corrector magnet 116) are usually adjusted first to achieve a highest possible ion beam current level in order to achieve a high efficiency for ion implantation production. Only after the ion beam current has been maximized, will the uniformity tuning start.
However, due to a number of deficiencies, existing methods for tuning an ion implanter system often fail to achieve a consistent ion beam output in an efficient manner. For example, existing algorithms for tuning an ion implanter system use a single channel to measure an ion beam current density profile a number of times. Such existing algorithms measure the same ion beam current density profile a number of times in parallel and then analyze these measurements. Therefore, many iterations of measurements may be required to properly tune an ion implanter system. Thus, existing algorithms render ion implanter systems less efficient by increasing tuning time and thereby decreasing productivity.
Existing tuning algorithms also tend to use ion beam current as the only criterion to optimize an ion beam. However, identical ion beam currents do not necessarily guarantee identical ion beam conditions. For example, identical ion beam currents can be produced with several different combinations of beam-line element settings. These different combinations often cause different ion beam dimensions, positions and angles. As a result, single-parameter approaches that rely solely on ion beam current can lead to inconsistent ion beam geometries. Further, due to different ion beam geometries, extra time must be spent, for each wafer batch, on ion beam measurement, parallelism setup, uniformity setup, and implant dose control.
Existing tuning methods further tend to rely on a single set of beam-line element settings recorded in a previously successful setup. Yet, many factors in an ion implanter system can change ion beam conditions even if beam-line element settings are maintained constant. For instance, an ion source usually has a lifetime during which ion generation gradually degrades. Therefore, even with identical beam-line element settings, ion beam current can be quite different depending on the length of time an ion source has been in use. Since a previously recorded single set of beam-line element settings often cannot reproduce a desired ion beam condition, an ion implanter system may have to be re-tuned for every wafer batch, which is quite time-consuming for reasons described below.
In view of the foregoing, it may be understood that there are significant problems and shortcomings associated with current ion implanter tuning technologies.